High-Resolution Opto-Electronic Retinal Prosthesis: Physical Limitations and Design - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 269

were cultured over this 3-layer structure for 7-14 in vitro, as well as implanted
into subretinal space in vivo, tissue was observed to migrate into the chambers
in both cases (Figure 14.7c).
Major concerns are whether the neural cells that migrate into the pores will
survive for an extended period of time, whether the neural circuitry will be
disrupted and whether the migrated tissue will change through glial overgrowth
or cell death. The long-term behavior of retinal cells migrating into perforated
membranes is currently under detailed investigation to optimize the membrane
structure for preserving neural connections and assuring efficacy of an electric
interface. It is important to emphasize though that for efficient stimulation
the target cells should be in proximity of the apertures, where the current
density is highest, but could be above the apertures rather than inside the
chambers.
Migration Around Protruding Electrodes
Another promising technique for providing close proximity between the neural
cells inside the retina and the stimulating sites of the implant involves protruding
electrodes. As diagrammatically shown in Figure 14.8a, stimulating electrodes
(1) would extend by several tens of micrometers above the surface of photodiodes
and be exposed only at the top of the “pillars,” with a common return electrode
(2) on the surface of the wafer. This array would be positioned in the sub-retinal
space, so that cells could migrate into the empty space between the pillars,
similarly to the migration we observed with the perforated membrane. This way
the electrodes will penetrate into retina without mechanical stress and associated
injury. The depth of penetration is determined by the length of the pillars and their
density. The pillars can be manufactured using conventional photolithographic
technology. An array with pillars of 70m in height and 10m in diameter
lithographically fabricated from SU-8 photoresist is shown in Figure 14.8b. Such
arrays having various pillar density have been implanted into the subretinal space
in adult RCS rats. Histology performed on eyes enucleated 15 days after the
implantation is shown in Figure 14.8c. As one can see in this figure, the retina
is well preserved with the pillars that penetrated into the inner nuclear layer.
On the left end, where the pillar spacing is wider, there is more volume for the
tissue migration, the retina rests lower and the pillars penetrate deeper into the
tissue.
Delivery of Information and Power to the Implant
Projection System
The projection system is designed to allow natural eye scanning for image
perception, flexibility of image processing between the camera and the implant,
and utilize any remaining natural vision and neural function. The system controls